Liquid-liquid extraction zones are extremely sensitive to fluctuations in flow rates and compositions of streams entering and leaving the zone. These fluctuations often result in process upsets which cause production of the desired products to be reduced or stopped, since the product streams are no longer at the desired compositions. Disturbances, or upsets, in an extraction zone cause upsets in the downstream equipment which purifies and further processes extraction zone product streams, thus adding to the problem of off-specification product and further aggravating the upset of the extraction zone by means of changes in reflux streams, if any, and changes in the solvent stream which is returned to the extraction zone for reuse after its separation from an extraction zone product stream. A common result of fluctuations is known to those skilled in the art as stacking, which is an increase in inventory of a substance in an extraction zone. The present invention is particularly adapted to mitigate stacking.
Commonly used automatic control schemes simply attempt to maintain all flows at constant values and assume that constant flows yield constant composition values. In conjunction with such automatic control systems, an operator must make adjustments in response to various fluctuations, including fluctuations in composition. However, such adjustments can only be made after a fluctuation is detected. Thus, fluctuations in flows and compositions of a magnitude sufficient to seriously affect an extraction zone are inevitable. Such fluctuations are frequently caused by performance of routine maintenance, flow rate changes, temperature changes and changes in charge mixture composition. As will be described below, a commonly used control scheme will actually aggravate certain problems rather than solving them, such as where the automatic controls cause raffinate to be removed from an extraction zone in a situation where raffinate should remain or be returned to the zone, in order to maintain relatively constant conditions in the zone, i.e., to maintain a constant composition profile.
Prior art methods of operation of extraction zones usually results in constantly changing compositions. As soon as a product composition, for example, moves off-specification, an adjustment is made. The adjustment causes the product composition to move toward the desired point and then to move away from it again in another direction. When the new excursion is detected, another adjustment is made. System parameters are continuously moving, within ranges whose span is determined by the quantity and quality of the corrective adjustments. It can be said that the system is constantly rocking back and forth. Thus the composition profile in the extraction zone is constantly shifting, increasing operating cost and lessening product quality.